Wireless networks are steadily growing and an increasing number of systems for mobile communications are deployed. For example, multiple standards of wireless cellular systems have been developed and introduced to the mass markets. While legacy 2nd generation cellular communication systems, like e.g. GSM (Global System for Mobile Communications), have been mainly introduced to meet traditional demands for circuit switched voice communications with comparatively low data rates, 3nd and 4th generation mobile communication systems, like e.g. UMTS (Universal Mobile Telecommunication System) and LTE (Long-Term Evolution), are getting increasingly complex to meet the ever increasing demands for higher and higher data rates needed for packet based mobile communications. Typically, different wireless communication systems of different generations employ different non-overlapping Radio Frequency (RF) bands, namely radio spectra, for communicating between different transceiver devices of the respective wireless communication system.
Recently, a demand for so-called multiband transceivers, e.g. multiband base stations, calls more and more attention by communication system providers. Thereby a multiband transceiver may be understood as a transceiver which is capable of transmitting and/or receiving RF signals in different spectral RF bands, preferably at the same time, i.e. simultaneously. The different RF signals may either belong to one communication system standard, i.e. they may all be compliant to the same wireless communication system. However, different RF signals transmitted by a multiband transceiver may additionally even belong to different communications standards, i.e. different transmitted or received signals may be compliant to the different wireless communication systems. Such multiband capable transceivers, which may simultaneously transmit/receive different signals in different RF bands, where the signals themselves may belong to different standards, are one of the most challenging issues for modern multiband base stations.
Physical implementations RF transmitters suffer from a number of impairments that degrade the quality of a received signal beyond the impact of fading or thermal-noise at a distant receiver. Analog components in one or a plurality of transmit chains, which will also be referred to as transmitter paths in the sequel, typically cause various impairments because of their imperfect behavior. Most prominent non-idealities are carrier-frequency and sampling-rate offset, phase-noise, IQ-imbalance (I=In-phase signal component, Q=Quadrature signal component), and Power Amplifier (PA) nonlinearities. Especially the mixer after a Digital-to-Analog Converter (DAC), which is responsible for mixing a baseband (BB) signal with a carrier signal, may cause so called IQ imbalance imperfections. Beside the mixer, the PA is another source of impairments. The non-linear behavior of the PA characteristic close to the saturation point adds nonlinear distortions to the transmit signal, too. The nonlinearity of the PA mainly results in sideband emission and affects spectrum emission requirements. In the sequel, the combination of all those impairments of a transmitter path will also be referred to as transmit noise (TX noise). The transmit noise may be interpreted as a deviation of a physical, i.e. real-world RF transmit signal from an ideal transmit signal constellation due to the aforementioned transmitter non-idealities.
The impact of TX noise may be at least partially compensated by classical algorithms. Separate/different linearization and compensation algorithms, that are particularly designed to solve these problems, may compensate at least some of the aforementioned TX impairments more or less accurately (depending on the algorithm), e.g. using feedback signal and signal processing in the digital part of the transceiver chain. For example, at the transmitter TX noise may be at least partially compensated by means of (digital) pre-distortion for which a feedback path from the output towards the input of a TX path is required.
In case of a multiband transceiver, which comprises a plurality of transmit paths for different target frequency bands in one physical package or housing, the TX noise and/or the feedback signals of a TX path may negatively influence one or more neighboring/adjacent TX paths dedicated to other frequency bands. Due to this cross-talk between different TX paths belonging to different RF frequency bands or even belonging to different wireless communication standards, an appropriate compensation of transmitter impairments or TX noise in the individual TX paths gets more difficult. Hence, it is desirable to improve this unsatisfactory situation for multiband transceivers.